BECKWITH ELECTRIC M-3425A, Instruction Book

Page: 227 / 482

4–96
M‑3425A Instruction Book
When 64B operates, indicating open brush
conditions, the 64F Function cannot detect a field
ground. For most generators, when the brushes of
the rotor shaft are lifted, the capacitance across the
field winding and the ground significantly reduces to
less than 0.15
m F. The 64B Function analyzes this
capacitance-related signal, and initiates an output
contact when it detects an open brush condition.
Typically, this output is used to alert operating
personnel of an open brush condition. Ranges and
increments are presented in Figure 4-71. The typical
pickup setting is listed in Table 4-9, Typical Brush
Lift-Off Pickup Settings.
In order to assure correct setting, it is recommended
that the actual operating value be predetermined
during the final stage of the relay installation. By
introducing a brush-open condition, the actual
value can be easily obtained from the relay. The
following procedure can be used to obtain the actual
operating value of the 64B during an open brush
condition:
8
WARNING: Machine should be off-line
and field excitation should be off during the
capacitance measurement.
64B PICKUP
mV
64B DELAY
cycles
64B/F FREQUENCY
Hz

NOTE : Field breaker should be closed for the
capacitance measurements.
1. After installation has been completed,
determine the rotor capacitance, as
outlined for the 64F function.
2. With the machine still off-line, apply
power to the relay and set the 64B/F
operating frequency in accordance with
the value listed in Table 4-8, Typical
Frequency Settings.
3. Introduce a brush-open condition by
disconnecting the rotor brushes or lifting
the brushes from their ground. Observe
the 64B voltage value displayed by
IPScom or the relay. The displayed value
is the actual measured operating value
of the 64B function.
4. To ensure correct operation and prevent
erroneous trips, the Pickup Setting for
the 64B Lift-off condition should be set
at 80–90% of the actual operating value.
The 64B/F Frequency is a shared setting common to both
the 64B and 64F Functions. If either function is enabled, this
setpoint is available, and should be set to compensate for the
amount of capacitance across the field winding and ground,
so that the measurement accuracy is improved.
To minimize measurement errors, the 64B/F frequency
should be set according to the amount of capacitance across
the field winding and the ground. Table 4-8 includes typical
settings of the frequency for capacitance, ranging from 1
m F to 10 m F.
Equivalent Brush Lift-Off
Capacitance
Typical Brush Lift-Off
Pickup Setting
0.05~0.25
m
F 2500 mV
Table 4-9 Typical Brush Lift-Off Pickup Setting